Johnson 120HP V4

[knuuttila]

Can anyone advise. I have a 120HP V4 which I,m sure is not running properly, although I cant pinpoint the problem, the engine just seems to be lacking power and I suspect one cylinder is not functioning properly(top righthand).<br /><br />When I put my hand on the right side of the engine, it feels much cooler than the left side. Is this a clue that one cylinder is not firing properly,or is this normal. I,ve checked the plugs, and they all seem similar, although again the right hand plugs are cooler than the left hand side.<br /><br />Are there any clever tips to try and find the problem(if indeed there is one)<br /><br />Many thanks<br /><br />Knuuttila

 

[Joe Reeves]

Re: Johnson 120HP V4

(Compression & Spark Test)<br />(J. Reeves)<br /> <br />1st - Regardless of what problem one might encounter, always start with the basic troubleshooting procedure. Check the compression which should be in the range of 100+ psi (somewhat less on the smaller hp engines) and even on all cylinders.<br /> <br />2nd - With the spark plugs removed, rig up some type of spark tester whereas you can set a gap to be jumped. On the solid state OMC ignition systems from 1973 to the present, set the gap to 7/16". The spark should jump the gap with a strong blue lightning like flame..... a real strong blue snap!

 

[DHPMARINE]

Re: Johnson 120HP V4

Joe is right and sending you in the right direction.Is the right side of the engine right when you are in the boat (facing stern) or outside the boat (facing towards the bow)?<br /><br />The starboard cylinders on your engine will feel hotter,because the water cools the port side first.<br /><br />DHP

 

[knuuttila]

Re: Johnson 120HP V4

Dear Joe/DHP, thanks for the reply.<br /><br />The compression is ok , but I,ll check the spark as you suggest.<br /><br />Looking at the engine from the boat, it,s the port cylinders that are hotter,<br /><br />Rgs<br /><br />Knuuttila

 

[Brew2]

Re: Johnson 120HP V4

To check if cylinder is making power....with motor running, pull the plug wire to each cylinder one at a time (use well insulated pliers). If RPMs drop when a wire is pulled, then that cylinder was making power. Replace the wire and move on to the next cylinder.<br /><br />If you pull a wire and have no change to RPMs then that cylinder is dead. Assuming you have good spark and compression, then I guess you'd be looking for a fuel related problem.<br /><br />Good Luck!